Standard Fracture Toughness Tests Research Articles

Analysis of compact tension specimens with deflected cracks for orthotropic materials

Anisotropic materials, such as alloy, wood and fiber-reinforced composites, are widely used in load-bearing components. Accurately obtaining its fracture performance is crucial for safety assessment. However, existing testing methods based on compact tension (CT) specimen have not taken into account material anisotropic characteristics and crack deflection. In this work, the systematic finite element analysis (FEA) was conducted for CT specimens with deflected cracks made of orthotropic materials. A wide range of geometric (crack deflection angle, β, and ratio of crack length to width, ap/W) and orthotropic material (λ and ρ) parameters were discussed. Complete solutions of the stress intensity factor (KI and KII) and load-line compliance (C) were determined for the first time. The results showed that the geometric dimensions and material parameters have a significant coupling influence on the fracture parameters. The influence of the λ is generally greater than that of the ρ. Changes of material parameters can make fracture parameters’ dependence on β vary. The variation of β and ap/W could enlarge or minish even dismiss the impact of λ and ρ. In addition, to further verify the importance of the obtained fracture parameters, the CT fracture tests of carbon fiber-reinforced epoxy resin under various orientations were conducted. The solutions will promote the optimization of the fracture toughness testing standards for CT specimens made of anisotropy materials.

Determination of the fracture toughness of carburized Pyrowear 53 steel for planetary gears by the small punch test method

The aim of the current study was to determine the fracture toughness of different zones in carburized Pyrowear 53 steel using the small punch test (SPT) method. Firstly, Pyrowear 53 steel was quenched and tempered using different processing parameters to obtain core materials with varied microstructures and fracture toughness. The results obtained for the core material in standard fracture toughness tests were then compared with the SPT results, which allowed the determination of a formula correlating the fracture energy integral, JSPT, from the SPT with JQ integrals obtained from standardized compact tension specimens. In the next stage, Pyrowear 53 steel was carburized at 925 °C and divided into the following zones: (1) a carburized layer (up to 0.5 mm from the surface), (2) a transition layer (from 0.5 to 1.5 mm), and (3) a core zone (more than 1.5 mm). Each zone was characterized in terms of its microstructure and tensile properties using miniaturized test specimens. Finally, the fracture toughness values of the core zone (JSPT = 78–102 kJ/m2), the transition layer (JSPT = 71–80 kJ/m2), and the carburized layer (JSPT = 8.1–9.1 kJ/m2) were determined based on the obtained SPT results. It was shown that the use of such a relatively simple SPT method with the proposed energy-based approach seems to be a promising way of determining the fracture toughness of thin layers or local changes in the fracture behavior of surface-treated materials.

Determination on the fracture toughness of the welded joints of X80 pipeline steels based on small punch test

For the sake of design and integrity assessment of the fracture toughness of high-grade pipelines, it is necessary to carry out accurate fracture toughness assessment of the welded joints. The study of more concise and effective fracture toughness testing methods has important theoretical and engineering value. At present, the fracture control of girth welding joints is mainly based on J integral, CTOD and CTOA. However, it is hard to carry out standard fracture toughness tests at some local narrow areas of the welded joints such as the heat affected zone for the difficulty of sampling. On this basis, this paper aims to propose a method to determine the fracture initiation toughness JIC of the welded joints of high-grade pipeline steels based on the micro-sample test method - small punch test. Firstly, small punch specimen with through-thickness notch was designed. Secondly, the finite element model of the notched specimen was established, and verified by the notched specimen tests of X80 steel. Furthermore, the elastic–plastic material properties of different zones of the welded joint were obtained by the empirical correlation method of small punch test and further used for the determination of J contour integral. The GTN damage parameters were calibrated by the three-point bending tests and further used for the determination of the crack initiation displacement. Finally, the fracture initiation toughness JIC(SPT) of different zones of the welded joint were obtained by the SPT method and verified by the three-point bending test. The results show that the fracture initiation toughness JIC(SPT) of each region of the welded joint obtained by notched SPT specimen were all less than the SENB test results. But there is a good empirical correlation between the two results so that the SPT method can be further modified by establishing an empirical correlation model. The SPT method has good engineering application prospect for the accurately characterization of the different distribution of fracture toughness in different areas of the welded joints.

Determination of fracture toughness through small punch test of notched-tube specimen

Fracture toughness is an important mechanical property for designing structures or for conducting safety assessments. In this study, determination methods of fracture toughness of P91 material were studied using finite element simulation based on the Gurson-Tvergaard-Needleman (GTN) model. Results indicated that the JIC of P91 could be exactly determined using the simulation of standard fracture toughness test based on the GTN model. When the small punch test (SPT) was used to determine JIC, a notched-plate specimen provided better results to determine the JIC of P91 compared to the other two methods. A notched-tube specimen of SPT was proposed to determine JIC of nuclear reactor fuel cladding tubes of P91. Simulation results indicated that the notched-plate specimen and the notched-tube specimen provided identical accuracy for determining JIC. Therefore, the determination method derived from the notched-plate specimen could be used to the notched-tube specimen.

A Comparative Study to Evaluate the Essential Work of Fracture to Measure the Fracture Toughness of Quasi-Brittle Material.

In the present work, three different woven composite laminates were fabricated using the hand lay-up method. The woven reinforcement fibres were carbon fibres (CFRP), glass fibres (GFRP-W) and (GFRP-R) in combination with epoxy resin. Then, the central notch specimen tensile test (CNT) was used to measure the fracture toughness and the corresponding surface release energy (). Then, the data were compared with the essential work of fracture (we) values based on the stored energy of the body to obtain a new standard fracture toughness test for composite laminates using relatively simple techniques. In addition to an extended finite element model, XFEM was implemented over a central notch specimen geometry to obtain a satisfactory validation of the essential work of fracture concepts. Therefore, the average values of () were measured with CNT specimens 25.15 kJ/m2, 32.5 kJ/m2 and 20.22 kJ/m2 for CFRP, GFRP-W and GFRP-R, respectively. The data are very close as the percentage error for the surface release energy measured by the two methods was 0.83, 4.6 and 5.16 for carbon, glass and random fibre composite laminates, respectively. The data for the fracture toughness of XFEM are also very close. The percentage error is 4.6, 5.25 and 2.95 for carbon, glass and random fibre composite laminates, respectively. Therefore, the fundamental work of the fracture concept is highly recommended as a fracture toughness test for composite laminates or quasi-brittle Material.

Fracture toughness examination of dual‐phase and interstitial free steel using essential work of fracture method

AbstractFracture toughness of automotive‐grade dual‐phase steel (DP450) and interstitial free (IF) steel is determined using essential work of fracture (EWF) methodology. Geometrical constraints of standard fracture toughness tests made the EWF method an alternative for thin sheets. Double edge notched tension (DENT) specimens are used for the EWF tests. The two steels have different mechanical and microstructural properties, which helps test the applicability of the EWF methodology to different materials. The effect of notch‐tip radius on the EWF test results is analyzed for both the steels. The sensitivity of fracture toughness to notch‐tip radius is not the same for both steels. During the EWF test, the digital image correlation (DIC) technique is used to determine the local strain variation of DENT specimens. The hole expansion ratio is determined for the two steels to compare against the EWF test results. The IF steel, despite lower strength, has better fracture toughness and hole expansion capability against the DP450 steel.

The Determination of the Limit Load Solutions for the New Pipe-Ring Specimen Using Finite Element Modeling

To estimate the acceptable size of cracks and predict the loading limit of the pipeline or its resistance to the initiation and crack growth by following the structural integrity, the fracture toughness and limit load solutions are required. Standard fracture toughness testing of thin-walled pipelines is often difficult to perform in order to complete standard requirements. To find an alternative technique for the measurement of the fracture toughness of the already delivered pipeline segment, the new pipe-ring specimen has been proposed; however, the limit load solutions have not been investigated yet. The limit load depends on the geometry of the specimen and loading mode. The ligament yielding of pipe-ring specimens containing axial cracks through the thickness under combined loads was calculated by the finite element method. This paper provides limit load solutions of several different pipe-ring geometries containing two diametric symmetrical cracks with the same depth ratio in a range of 0.45 ≤ a/W ≤ 0.55. The limit load (LL) solutions calculated by numerical analysis are shown as a function of the full ring section’s size and the corresponding crack aspect ratio for determining the normalized load. These can potentially construct the failure assessment diagram to estimate the crack acceptance in a part of the pipe.

Reliability research of reference temperature estimation of RPV materials using Charpy impact energy

The standard fracture toughness test is usually limited in practical engineering projects due to the expensive and sophisticated experimental procedure or limited specimens. The traditional method to evaluate the structural integrity is to estimate the ductile–brittle transition region by Charpy tests. However, this fuzzy estimation is sometimes too conservative and cannot obtain quantitative fracture toughness data. In recent years, many achievements have been made in the study of the relations between Charpy impact test and fracture toughness test. Although acceptable, the errors caused by these formulas are rarely compared in detail formulas with stable deviation will be more suitable for research or engineering application. Therefore, this paper compares the data of four different ferritic steels to test the error stability of each formula, so as to provide reference for experimental research and engineering facilities. The reliability of each correlation is checked from the view of accuracy or appropriate conservatism. In this research, We found that the T28J–T0 correlations, the empirical formula proposed by Rolfe, Novak and Barsom (abbreviated as RNB) and the Mean-4 Procedure obtained by IGC-parameter give the reference temperatures more accurately or appropriately conservatively. These formulas should be preferred in practical application.

Experimental verification to determine fracture toughness from the small punch test using “Local approach”

The main tasks of the small punch (SP) technique are to derive standard material properties from a limited amount of material, including strength/toughness, creep and fatigue behaviour etc. The complicated problem of determining fracture toughness poses significant difficulties. Although many methods have been put forward to increase the prediction accuracy, no satisfactory methods are available yet. Recently, the so-called “Local approach” has been introduced to determine ductile fracture from the small punch test by means of a micro-mechanical model introduced by Gurson and modified by Needleman and Tvergaard. By using the observed load-deflection curve from the small punch test, reverse finite element analysis is performed to identify both the material plastic property (σ-ε curve) and the damage parameters of the Gurson model. With these material parameters known, a standard fracture toughness (CT) specimen is simulated by finite element analyses, from which the J-integral and the crack extension can be estimated. A J-R resistance curve can be created by multiple specimens with different load-line-displacements, and the fracture toughness JIC can be determined according to ASTM E 1820-17.In order to verify this new approach, the standard uniaxial test, the small punch tests and the standard fracture toughness tests are needed with the same material. Up to now, only a few of these kinds of experimental verification are available. In this paper, an overview on the existing methods of determination of fracture toughness from small punch test is given, and the “Local approach” for small punch test is addressed. Verifications are summarized from three high level institutes, including MMT in the Czech Republic, JRC Petten in the Netherlands and EPRI in the USA. The predicted JIC with that from standard fracture toughness tests are compared. A good agreement is found and shows that the “Local approach” is capable of predicting fracture toughness from small punch tests. The prediction accuracy by “Local approach” is better than existing methods. Further work is needed as available tests are still limited.

Determination of Material Fracture Toughness by Circular Pre-Cracked Small Punch Test Specimens

Fracture toughness empirical correlation between SPT(Small Punch Test) with non-crack sample and standard fracture toughness test has been established in recent years. In order to compensate the imperfection of empirical correlation, such as absence theoretical basis, poor repeatability and universality, in this paper, an O-type pre-cracked sample was adopted to evaluate fracture toughness. The mechanical model of the sample is in compliance with plane strain condition in the direction of crack propagation. In this paper a determination procedure was studied and established, and the J-integral of steel Q345R was calculated using the procedure.

Fracture toughness for engineering application: There is a need for more suitable testing standards

Fracture toughness testing standards such as ASTM E1820 or ASTM E 1921 ISO 12350 are seldom used in practice. Estimates of KIc or JIc by empirical correlations with classical Charpy impact tests are much more popular and often preferred. The reasons for this unsatisfying situation is that the standard procedures of fracture toughness testing are much more demanding than the Charpy test, in terms of costs, test material, test equipment and expertise of the laboratory personal, whereas the much simpler CV-test still delivers useful estimates. However, between standard fracture toughness testing and classical Charpy impact testing there is a wide space for possible new tests that are similarly simple as Charpy tests and similarly informative as a fracture toughness tests. In the present paper, some suggestions for such more practical fracture toughness tests are made.

Analysis of standard fracture toughness test based on digital image correlation data

This paper deals with an experimental methodology based on the 2D Digital Image Correlation method for precise full-field in-plane stress-strain analysis of a region containing a crack. The presented methodology is employed as a supporting and comparative method for the standard J-integral testing defined by the ASTM. The methodology utilizing the DIC-based displacements for a subsequent evaluation of strain, stress and final extraction of the J-integral from its definition. This J-integral is compared with the value obtained by the ASTM standard and with the value obtained by the DIC-based CTOD measurement. Several techniques improving the conventional DIC and data post-processing are proposed to achieve reliable full-field strain/stress results. A direct comparison between the experimental and FEM full-field results is carried out.

CVN–KJC correlation model for API X65 gas pipeline

Abstract Reliable determination of fracture toughness of different materials is critically important. However, standard fracture toughness testing methods are difficult and costly. In order to simplify the determination of fracture toughness, several correlations between fracture toughness and Charpy impact energy (CVN) have been proposed by different researchers. In the present work, a new linear model was developed for estimation of fracture toughness (KJC), based on CVN, and yield strength data, in base, seam weld, and girth weld metals of an API X65 gas pipeline. The predicted values of KJC from the suggested relation, with maximum and mean error values of 6.7% and 4.4%, respectively, was found to be in good agreement with the valid fracture toughness test results obtained from compact tension specimens following ASTM E1820 standard.

Modification of fracture toughness Master Curve considering the crack-tip Q-constraint

Engineering structures may have significant lower crack-tip constraint compared with those in the standard fracture toughness test specimens. This paper considers a methodology to extend the application of fracture toughness master curve approach for structures with low crack-tip constraint. This is conducted by considering the Q-parameter as a suitable crack-tip constraint parameter, and study its effects on the reference temperature T0 in the master curve approach. The experimental fracture toughness data from the literature as well as supplemental experiments are used for this purpose. The relation between Q and T0 is investigated by using numerical analyses of experiments covering various types of geometry, loading condition, thickness and crack depth. The effects of loading amplitudes and material properties on the relation between Q and T0 are also studied. Based on this study, a relationship between Q and T0 is derived and experimentally verified. It is shown that the proposed relationship has a considerable ability to predict the apparent fracture toughness behavior in the ductile to brittle region of complex structures without need of conducting additional experimental fracture toughness tests.

Estimation of Fracture Toughness of Metallic Materials Using Instrumented Indentation: Critical Indentation Stress and Strain Model

We propose a generalized approach based on fracture mechanics and contact mechanics to estimate the fracture toughness in metallic materials from instrumented indentation testing. Models were developed for brittle and ductile fracture. Different criteria were applied to each model to determine the critical fracture point during indentation. For brittle fracture, the critical fracture point was defined in terms of the critical mean pressure; for ductile fracture, the critical fracture point was derived from fracture strain and critical plastic zone size. Each fracture criterion was used to determine the indentation fracture energy corresponding to the fracture energy required for crack extension. The fracture toughness was estimated for various metallic materials using each model and compared with standard fracture toughness tests.

Cold rolling improves the fracture toughness of a Zr-based bulk metallic glass

Mode I fracture tests were conducted on both as-cast and cold-rolled Zr-Cu-Ni-Al-Nb bulk metallic glass (BMG) single edge notch bend samples. The results show a 54% improved fracture toughness (KJ = 55.2 vs. 85.0 MPa√m) is obtained by cold rolling to ∼2.5% true strain. The precrack patterns and fractography were also analyzed to evaluate the influence of cold rolling on the fracture process and also the validity of the mode I fracture toughness results. Cold rolling was found to promote numerous shear bands along the main crack, crack bifurcation, and a tortuous crack path, all of which are thought to contribute to the improved toughness. The influence of various precrack morphologies on the fracture toughness results are discussed. It was found that it is difficult to produce perfect standardized precracks in BMGs with heterogeneous glassy structures induced by cold rolling and this issue needs further evaluation if fracture toughness testing standards for BMGs are to be established.

International round-robin test on an improved indentation fracture (IF) method performed through high-magnification microscopy with a traveling stage

An international round-robin test on an improved indentation fracture (IF) method was conducted successfully with seven laboratories using solid-phase sintered silicon carbide, silicon nitride, and alumina. An objective lens with a high magnification of either 40× or 50× and a calibrated traveling stage were employed to enable both clear identification of the crack tips in high resolution and precise measurement of the crack length. Misreading of the crack length by each laboratory was reduced compared with the conventional IF method, resulting in a moderate scatter in the indentation fracture resistance (KIFR) data collected by the participating laboratories. The coefficient of variance for the three samples was less than 8%, which is comparable to those typical of standard fracture toughness tests such as the surface crack in flexure method and the single-edge V-notch method. Additionally, the results were consistent with those of our previous domestic round robin test, thus confirming the reliability of the improved IF method both internationally and domestically.

Estimation of the Fracture Toughness of Structural Steels by Means of the CTOD Evaluation on Notched Small Punch Specimens

By means of the experimental analysis of two structural steels, one with a ductile behaviour and the other one with brittle behaviour, this paper compares the results obtained by means of small punch test (SPT) performed on notched samples with those obtained in standard fracture toughness tests, in order to obtain a relationship between them and to analyse the suitability of the SPT for estimating the fracture toughness. With the aim of analysing the evolution of the defect tip opening displacement (δSPT) during the test, loading was interrupted at different levels of punch displacement (d) and these specimens were analysed under SEM.In the case of the ductile steel, δSPT at the onset of crack growth was larger than the value of CTOD obtained in conventional fracture tests, but this could be expected due to the lower constraint condition of the SPT. By the other hand, the brittle steel exhibits ductile fracture micromechanisms in the SPT, so fracture mechanisms have changed from the standard to the small punch test, due to the different load conditions. Relationship between d and δSPT seems to be material independent.

A load–deformation formulation with fracture representation based on the J–R curve for tubular joints

This study presents a new fracture formulation to describe the ductile tearing and unstable fracture failure for circular hollow section (CHS) joints under monotonically increasing brace tension. The initiation of the ductile tearing occurs when the crack driving force in an assumed initial shallow crack reaches the material fracture toughness determined from a standard fracture toughness test. The joint behavior prior to the ductile crack initiation follows a previously proposed nonlinear formulation based on the latest strength equations recommended by the International Institute of Welding. The load–deformation characteristics beyond the crack initiation assume that the energy release rate and the amount of crack extension adhere to the experimentally measured J–R curve, prior to the unstable fracture failure. Unstable fracture, which leads to the total loss of the joint capacity, occurs when the crack driving force reaches the maximum fracture resistance determined from the J–R curve test. The proposed load–deformation representation for tubular joints, when implemented in the large-scale K-frame pushover analysis with a material fracture toughness test, predicts successfully the global frame response governed by the joint fracture failure, as observed in the frame test.

Dynamic fracture toughness of Al–7Si–Mg (A357) aluminum alloy

Impact behavior related to crack initiation and growth of cast A357 (Al–7Si–0.7Mg) alloy was experimentally investigated. Dynamic fracture toughness for various artificial aging conditions were evaluated by exploiting instrumented impact Charpy V-notch experiments; impact tests were preferred instead of the standard fracture toughness tests in order to have a fast assessment of the effect of artificial aging on the fracture toughness behavior. Characteristic points of impact load–deflection curves related with cracking were identified and post-correlated with different artificial aging conditions. Absorbed impact energies for stable and unstable crack growth were calculated for 28 different aging conditions; resulting cracking velocity at the unstable crack regime (rebound compliance) was also calculated and it was found that rebound compliance is inversely related to ductility. Cracking initiation load values were identified by exploiting the compliance changing rate method; both stress intensity factor and J-integral analysis were employed to calculate critical dynamic fracture toughness values. They were found to be slightly higher than corresponding quasi-static properties and they are both changing with varying aging conditions. Limitations and results of both notch-fracture toughness analyses were discussed by taking into account the material aging condition. Performed scanning electron microscopy on the fracture surfaces revealed the fracture mechanism that is correlated with respective microstructure.